1. Technical Field
The present invention relates to a composite semipermeable membrane for separating a component of a liquid mixture selectively, a method for producing the same, and a water-treating method using the composite semipermeable membrane. The present invention particularly relates to a composite semipermeable membrane comprising a thin film made mainly of a polyamide on a porous base material and having practical water flux, desalting faculty and endurance, and a water-treating method using the composite semipermeable membrane.
2. Technical Background
As semipermeable membranes used for purposes described above, there are known asymmetrical membranes wherein asymmetrical structures are made of the same material by a phase-separating method and composite semipermeable membranes wherein a thin film which is made of different materials and has a selective separability is formed on a porous base material.
As the latter semipermeable membranes, suggested are a great number of composite semipermeable membranes wherein a thin film made of a polyamide obtained by interfacial polymerization of a polyfunctional aromatic amine and a polyfunctional aromatic acid halide is formed on a porous base material (for example, JP-A Nos. S55-147106, S62-121603, S63-218208, H2-187135, and so on). Suggested are also composite semipermeable membranes wherein a thin film made of a polyamide obtained by interfacial polymerization of a polyfunctional aromatic amine and a polyfunctional alicyclic acid halide is formed on a porous base material (for example, JP-A No. S61-42308, and so on).
In order to improve the water flux of the above-mentioned composite semipermeable membranes further, additives are suggested. There are known substances capable of removing hydrogen halide generated by interfacial reaction, such as sodium hydroxide or trisodium phosphate; known acylating catalysts; compounds for decreasing the interfacial tension on a reaction field at the time of interfacial reaction; and so on (for example, JP-A Nos. S63-12310, H6-47260, H8-224452 and so on).
For these semipermeable membranes, endurance such that various oxidizers can be resisted, in particular, washing with chlorine can be resisted is demanded in light of more stable operability in various water treatment plants, a typical example of which is a water-producing plant, and pursuit of low costs based on prolongation of the lifespan of the membranes. It is said that the polyamide-based semipermeable membranes exemplified above have practical oxidizer resistance. It is not, however, said that all of them have resistance having such a level that constant or intermittent chlorine-sterilization can be resisted for a long time. It is therefore desired to develop semipermeable membranes having both of a higher oxidizer resistance and practical water flux and desalting faculty.
For these purposes, suggested are a composite membrane obtained from a diamine having only a secondary amino group (JP-A No. S55-139802), a composite membrane obtained using an aliphatic diamine or alicyclic diamine (JP-A Nos. S58-24303, S59-26101, S59-179103, H1-180208, and H2-78428), a composite membrane having a diphenylsulfone structure (JP-A Nos. S62-176506, S62-213807 and S62-282603), a membrane to which a chlorine-resistance is given by post-treatment (JP-A No. H5-96140), and so on.
However, these membranes do not have water flux, desalting faculty or oxidizer resistance which are required for practical semipermeable membranes. Thus, higher properties are demanded. In other words, it is known that in polyamide based reverse osmotic membranes, polyamides obtained using an aliphatic diamine whose main chain does not any aromatic ring are superior in oxidizer resistance, as described above, but the desalting faculty and water flux of the reverse osmotic membranes are not sufficiently satisfied.
The above-mentioned JP-A No. H1-180208 discloses a production process comprising the step of immersing a polyamide based composite semipermeable membrane obtained using both a polyfunctional aromatic amine and an aliphatic diamine into an aqueous chlorine-containing solution having a pH of 6 to 13. However, the publication never suggests what kind of other composite semipermeable membranes this process can be applied to.
Thus, an object of the present invention is to provide a composite semipermeable membrane having both practical water flux and superior desalting faculty and oxidizer resistance, a method for producing the same, and a water-treating method which makes it possible to exhibit practical water flux and superior desalting faculty and oxidizer resistance, using the same.